The present invention relates generally to an optical communications apparatus and method and, more particularly, to a free space optical communications apparatus and method that may be employed to transmit information long distances through the atmosphere.
Free space optical communications are of considerable interest for satellite-to-satellite communications (satellite crosslinks) since the region between satellites (the communications channel) is ideal, essentially a vacuum, which does not distort or attenuate the light beam. For earth-to-satellite and satellite-to-earth communications, on the other hand, the Earth""s atmosphere strongly influences the light transmission between information source and the destination. In such cases, (except for very short transmission paths) the transmission reliability of free-space optical transmission systems is considered unsatisfactory for most communications purposes.
There does not exist in the art a free-space optical communications system that can transmit data between the earth and a satellite, or that can otherwise transmit data long distances through the atmosphere.
The present invention comprises a free-space optical communications system capable of transmitting data over long distances through the atmosphere. The communications system according to the present invention comprises an optical transmitter at a first location, the optical transmitter comprising a femtosecond pulsed laser source for producing a train of femtosecond laser pulses having a pulse width in the range of from about 1 attosecond to about 1000 femtoseconds. The optical transmitter further comprises a laser pulse width modulator to provide a pulse width modulated train of femtosecond laser pulses corresponding to the data being transmitted. The communications system according to the present invention further comprises an optical receiver at a second location for receiving the modulated train of femtosecond laser pulses. The optical receiver comprises a spectral analyzer for receiving and determining the spectral components of the pulse width modulated train of femtosecond laser pulses. The optical receiver further comprises decoder circuitry operable to replicate the transmitted data based on the spectral components, i.e., frequency or wavelength content, of the modulated train of femtosecond laser pulses. It has been found in accordance with the present invention that the spectral characteristics of the femtosecond pulses of given initial pulse width are retained, even where considerable pulse width broadening has occurred upon transmission. In one embodiment, the optical communications system according to the present invention is adapted for transmission through the atmosphere, such as an earth-earth communication link, including long distance earth-earth links, and earth-satellite communication link. In another embodiment, the optical communications system is adapted for transmission through a media which is essentially a vacuum, such as a satellite-satellite communication link. In still another embodiment the transmission medium may be a fiber optic link.
In another aspect, a method for optically transmitting data employing the optical communication system according to the present invention is also provided.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate an embodiment of the invention and together with the general description, serve to explain the principles of the invention.